I discovered that if you're using a macbook that has the newer trackpads that support multi-touch, you can use the two-finger spread gesture to zoom in while in full screen mode in preview.
However, I can't figure out how to replicate this functionality with the keyboard or via menus, which means I can't use it on my older MBP... anybody know if this is even possible?
I don't know why they'd hide this functionality, but it's certainly not immediately obvious to me.